关键词: 雄安新区, 土壤气体通量, Rn, CO₂, 浓度, 容城断裂, 牛东断裂

Abstract:

Based on the analysis and processing of field mobile observation data, soil gas Rn and CO₂ degassing characteristics in the main fault zones of Xiong'an New Area(XNA)and its effects on the regional environment was preliminarily studied. Repeated observations of soil gas concentrations and fluxes in 2020 and 2021 for the seven measurement lines on the buried faults show that strong degassing characteristics exist in the main fault zones of XNA.
The range of variation of the mean soil gas Rn fluxes for each profile is from 71.44 to 335.35mBq/m²·s, and the range of variation of the mean CO₂ fluxes is from 25.96 to 78.23g/m²·d; the range of variation of the mean Rn concentration intensity is from 0.91 to 2.30, and the range of variation of the mean CO₂ concentration intensity is from 1.13 to 2.61, which are comparable to the degassing intensity of soil gas in other typical fault and earthquake zones in the world. Except for the flux of Rn in the Niudong branch fault 1 and CO₂ in the Niudong branch fault 2, the average values of Rn and CO₂ fluxes in 2021 are higher than those in 2020.The maximum variation of soil gas Rn flux is 116%in the Niudong Fault, and the maximum variation of CO₂ flux is 370%in the Niudong Fault; the variation of soil gas Rn concentration intensity is 119%in the Niudong Fault, but there is no significant variation of concentration intensity in other faults in both periods.
The observation and analysis found that the areas of high soil gas concentration anomalies on the three seismic profiles in XNA are highly coincident with the distribution of deep faults, showing a concentrated degassing phenomenon along the fault zones. The AA' seismic profile on the west side exposes three hidden fractures, which are the Taihangshan Fault, Rongcheng Fault, and one unnamed fault on the west side. The AA' soil gas Rn and CO₂ concentration profiles, corresponding to the location of the upward trend line of the Rongcheng Fault and the unnamed fault on the west side, show the characteristics of simultaneous single-peak type high-value anomalies of Rn and CO₂ concentrations. This phenomenon may indicate that the Rongcheng Fault and the unnamed fault on the west side are still highly active.
The BB' soil gas profile shows two areas of high soil gas concentrations respectively in the east and west. The western high value area shows synchronous peak anomalies of Rn and CO₂ concentrations, and the location of the anomalies is basically consistent with the upward extension direction of the Xushui-Dacheng Fault. In the east, only single-peak anomalies in CO₂ concentration are observed, and the magnitude of the anomalies is more significant than that in the western section, but there is no corresponding fault. Therefore, the synchronous peak anomalies of Rn and CO₂ concentrations in the western section should be related to the fault activity of Xushui-Dacheng Fault, while the single-peak anomalies of CO₂ concentrations in the eastern section may be non-tectonic.
The CC' seismic profile contains the Taihangshan Fault, Niudong Fault, Niudong branch fault 1, Niudong branch fault 2, and several secondary faults. The peak anomalies of soil gas Rn concentration in the soil gas profile are detected at different degrees and near the locations corresponding to the upward extension trend lines of Niudong Fault, Niudong branch fault 1, and Niudong branch fault 2. All three peak anomalies of soil gas Rn concentration may be related to the activities of Niudong Fault, Niudong branch fault 1, and Niudong branch fault 2. However, the single-peak CO₂ concentration anomaly is detected only above the Bazhou Depression, and the anomalous area basically overlaps with the area where the Bazhou Depression is located. The Bazhou Depression is one of the main areas of concentrated population in XNA, and the biological activity is relatively strong. According to the results of the existing study that soil gas CO₂ generated by biological activities may also be the main recharge source of CO₂ gas released from fault zones in the basin, the large-scale CO₂ concentration anomalies detected above the Bazhou Depression may also be generated by biological activities in the basin.
The results show that strong degassing characteristics exist in the main fault zones of XNA. The environmental effects of gas release from the main fault zones in XNA are evaluated by combining with the comprehensive prevention and control standards for indoor gas environmental pollution. The highest value of radon gas release in the main fault zone of XNA reaches 675mBq/m²·s in the Rongcheng Fault, and 395.70mBq/m²·s and 334.84mBq/m²·s in the Nudong branch faults respectively, the results indicate that it is necessary to carry out comprehensive radon prevention treatment for the buildings above the Rongcheng fault and Nudong fault zone. The preliminary estimation results of CO₂ release show that the daily contribution of CO₂ degassing from the main fault zones of XNA to the atmosphere is about 1 622.56t, and the annual contribution is as high as 0.59×10⁶t. Attention should be given to its impact on the regional environment.
The research results in this paper are significant for urban planning, environmental management and comprehensive assessment of the environmental impact of gas release from the fault zone in XNA.

Key words: Xiong'an New Area, soil gas flux, Rn, CO₂, concentration, Rongcheng Fault, Nudong Fault